Opposition candidate Park Won-soon must apologize for allowing leftover pesticide levels in school meals to exceed safety limits, the ruling Saenuri Party’s Seoul mayoral candidate Chung Mong-joon said in a press conference in front of Seoul City Hall on Thursday.

Park, as Seoul mayor in 2013, had known pesticide residue in school lunches were problematic and did nothing to address the issue, Chung said.

Park of the main opposition New Politics Alliance for Democracy has denied the accusation, saying he was never notified by auditors.

Chung’s charge is based on a 2013 report by the Board of Audit and Inspection. Seoul’s public schools served meals with “excessive pesticide residue amounts” to students, auditors had written in the report.

“Park first denied the charge. He then finally acknowledged the reports. Now, he’s saying he never knew about them and is dodging responsibility,” Chung said. Former lawmaker and 2011 Seoul mayor candidate Na Kyung-won also attended the conference, saying she was there as the mother of a student who may have eaten the contaminated meals.

Park has said the statements were printed as “footnotes” in the auditing reports and authorities “never told him” about the issue.

“We’re the ones who asked for auditors to check our school meals in the first place. When auditors explained their findings, they never mentioned pesticides,” Park said. A spokesperson for Park’s campaign team later conceded there had been two cases of excessive leftover pesticide levels, after reviewing the report.

Pesticide-infected school meals have become a campaign issue with less than a week left until voters pick Seoul’s next mayor. Chung trailed Park by more than 10 percentage points in multiple polls taken over the past week. The seven-time lawmaker has touted school meals in efforts to cut Park’s lead, which was widened further in the wake of the Sewol disaster on April 16. Chung’s approval ratings continued to slide to new lows after the ferry sinking, finally rebounding to 39.6 percent in a poll taken this week. Park received a 50.5 percent voter approval rating in the same poll conducted by Research & Research on Monday and Tuesday.

Prosecutors raided Seoul Agro-Fisheries & Food Corp. distribution centers in Seoul on Wednesday after the auditor reports in question became controversial in the Seoul mayoral race. Prosecutor General Kim Jin-tae has temporarily stopped the investigation until after the local elections to prevent “needless misunderstandings that the prosecution is taking political sides.”